In court filings, Mueller said Flynn, a former aide to President Donald Trump, had played a significant role in helping the investigation and therefore a sentence "at the low end of the advisory guideline range is appropriate and warranted".

The recommendation by Mueller comes almost two weeks before Flynn's sentencing, on December 18.

The documents, consisting of a sentencing recommendation and a partially redacted addendum to protect ongoing investigations, state that Flynn was an important cooperator after Mueller's team reached a plea deal with the former lieutenant general, who pleaded guilty to lying to investigators in December 2017.

"The defendant's decision to plead guilty and cooperate likely affected the decisions of related firsthand witnesses to be forthcoming," the court filings said.

"The defendant deserves credit for accepting responsibility in a timely fashion and substantially assisting the government," it added.

Following his plea deal, Flynn participated in 19 interviews with Mueller's team and provided documents and communications helpful to the investigation.

Not only did Flynn help investigators with their inquiry into ties between the Trump team and Russian operatives, but he also provided information on two other criminal investigations, according to the document.

The details of one of those investigations were completely redacted in the court filings.

Lying to the FBI

Last year, Flynn pleaded guilty to "willfully and knowingly" making "false, fictitious and fraudulent statements and representations" to the FBI in 2016.

Following the election of Trump, Flynn was appointed National Security Adviser, one of the more senior roles in the White House following Trump's election victory.

In his guilty plea, however, he said he "inadvertently briefed the vice president-elect and others with incomplete information regarding [his] phone calls with the Russian ambassador".

Those conversations between Flynn and then-Russian ambassador Sergey Kislyak happened on the day the outgoing Obama administration imposed sanctions on Russia.

Flynn had initially told Trump advisers that he did not discuss those sanctions, but he later told White House officials that he may have discussed sanctions with Kislyak.

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The former National Security Adviser, who resigned on February 13 2016, only weeks after Trump's inauguration, also lied about registering as someone working for the Turkey government under the Foreign Agents Registration Act (FARA).

Flynn did not register, however, and in this role, he and his company received $530,000 for trying to influence public opinion about the failed coup attempt in Turkey.

In this role, Flynn wrote an op-ed for a major US publication in which he called for the US to support Turkey, but he failed to disclose he received money from Turkey to do so.

'Witch-hunt'

So far, a total of 36 people have been indicted in the Mueller investigation, which was started in May 2016 to investigate links or coordination between the Russian government and individuals associated with the Trump campaign.

Flynn is one of five Trump aides who has pleaded guilty so far in the Mueller investigation.

The other Trump associates are former campaign manager Paul Manafort, former campaign aide Rick Gates, foreign policy adviser George Papadopoulos and Trump's personal lawyer Michael Cohen, who pleaded guilty last week during a surprise appearance in court.

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Besides the five Trump aides, others that have been charged so far are Russian-Ukrainian political adviser Konstantin Kilimnik, Dutch lawyer Alex van der Zwaan, US lobbyist Richard Pinedo and 25 alleged Russian intelligence officers and three Russian companies directly involved in trying to meddle in the election.

Trump has so far denied all allegations made against him and his campaign, often calling the investigation a "witch-hunt" despite the 36 charges brought forward so far.